1. Field of the Invention
The present invention relates to an image processing apparatus, an inkjet printing apparatus and a image processing method, and more particularly, to technology that suppresses bronzing in a printed image using what is called clear ink, which substantially does not contain color material.
2. Description of the Related Art
A technology for suppressing bronzing is known in which the bronzing is reduced by controlling the usage amount of color inks, as described in Japanese Patent Laid-Open No. 2001-138555. Specifically, whether or not bronzing will occur is determined based on the lightness of the image to be printed and the ink usage amounts, and bronzing is suppressed by changing the ink usage amounts according to the determination result.
Typically, when observing printed material, an observer observes light that was radiated onto the printed material from a light source and then reflected from the printed material. The observed color differs according to the observation angle. FIG. 1 is a diagram for explaining the difference in two types of reflected light in the case of observing printed material from two different angles. In FIG. 1, the reflected light observed in the direction A is specular light as observed from a direction of specular reflection with respect to the light source, and an image of the light source appearing on the printed material is observed. On the other hand, the reflected light observed in the direction B is reflected light that passed through the printed material and reflected out, or in other words diffuse light, and a color reproduced by the color material is observed. Also, in the case of observing the printed material from the direction A, the light source may be recognized as appearing in a different color from the color of the light source. This is perceived as bronzing.
The method described in the above Japanese Patent Laid-Open No. 2001-138555 has the following problems when observing such printed material. Different tints in the two types of observed light illustrated in FIG. 1 provide feeling of strangeness to the observer. Typically, printing with cyan ink causes a comparatively large difference in the tints in the two types of observed light, and the perception of the specular light becomes a red color different from cyan. In this case, when reproducing the high saturation color of cyan that is one of three primary colors, substituting this cyan with ink of a different color is difficult. For this reason, the technology of changing the usage amount of color inks described in Japanese Patent Laid-Open No. 2001-138555 cannot be applied.
Regarding differences in the tint of observed light as above, the use of clear ink which does not contain color material is known as a technology enabling a reduction in the tint differences of specular light while suppressing bronzing. FIGS. 2A to 2D are diagrams explaining the principle of suppressing bronzing by applying clear ink to printed material after image formation. FIG. 2A illustrates how light reflects normally off the surface of the colored ink layer. Note that for the sake of simplicity, light that is transmitted through the colored ink layer and reflected off surface of the print medium has been omitted. In contrast, FIG. 2B illustrates the case of forming a uniform clear ink layer over the colored ink layer. There exist light 1003 that reflects off the surface of the clear ink layer 1001, and light 1004 that passes through the clear ink layer 1001, reflects off the surface of the cyan colored ink layer 1002, and exits out from the clear ink layer 1001. The light path length of the light 1004 is longer than the light 1003 by a distance equivalent to passing through the clear ink layer 1001. The shift in optical phase based on the light path difference strengthens or weakens the intensity at a specific wavelength to produce light interference, and the tints of bronzing become different. Utilizing this phenomenon, changing the clear ink use amount can vary the coverage of clear ink over the colored ink layer and the thickness of the clear ink layer (see FIGS. 2C and 2D). Thus, the light interference state can be varied to control hues of bronzing color.
However, even though it may be possible to control the optical interference state and bronze hues to some extent by modifying clear ink application amounts, the problem of color change due to bronze colors and the optical interference state related to bronzing may not be resolved in some cases.
More specifically, the optical interference state and bronze colors are caused by further enhancing and blurring color formed by colored ink. This is because colors specified as the same color in the gamut are actually perceived as different colors by an observer. As a result, if image data is generated on the basis of just the colorimetry values associated with the gamut as in the related art, and an image is printed on the basis of such image data, the observer may perceive unnatural color differing from the expected color. In this case, if the application amounts of clear ink and ink or the number of scans by a print head for completing printing on a unit area differ among a plurality of print modes, differences will occur in the way that the clear ink functions and the degree to which clear ink suppresses the above color change due to bronze colors and the optical interference state. As a result, the image colors actually perceived by the observer will appear differently among different modes.